EP0571211B1 - Bobine asymétrique de radio fréquence pour l'imagerie par résonance magnétique - Google Patents

Bobine asymétrique de radio fréquence pour l'imagerie par résonance magnétique Download PDF

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Publication number
EP0571211B1
EP0571211B1 EP93303931A EP93303931A EP0571211B1 EP 0571211 B1 EP0571211 B1 EP 0571211B1 EP 93303931 A EP93303931 A EP 93303931A EP 93303931 A EP93303931 A EP 93303931A EP 0571211 B1 EP0571211 B1 EP 0571211B1
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EP
European Patent Office
Prior art keywords
coil
radio frequency
elements
reactive
loop
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
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EP93303931A
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German (de)
English (en)
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EP0571211A1 (fr
Inventor
Perry Scott Frederick
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General Electric Co
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General Electric Co
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
    • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
    • G01R33/36—Electrical details, e.g. matching or coupling of the coil to the receiver
    • G01R33/3692—Electrical details, e.g. matching or coupling of the coil to the receiver involving signal transmission without using electrically conductive connections, e.g. wireless communication or optical communication of the MR signal or an auxiliary signal other than the MR signal
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
    • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
    • G01R33/34—Constructional details, e.g. resonators, specially adapted to MR
    • G01R33/34046—Volume type coils, e.g. bird-cage coils; Quadrature bird-cage coils; Circularly polarised coils
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
    • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
    • G01R33/36—Electrical details, e.g. matching or coupling of the coil to the receiver
    • G01R33/3628—Tuning/matching of the transmit/receive coil

Definitions

  • the field of the invention is nuclear magnetic resonance imaging systems; and more particularly, to coils employed in such systems to excite the nuclei and receive electrical signals produced by the nuclei.
  • any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant ⁇ of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
  • polarizing field B 0 When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B 0 ), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M z is produced in the direction of the polarizing field, but randomly oriented magnetic components in the perpendicular, or transverse, plane (X-Y plane) cancel one another.
  • the net aligned moment (M z ) may be rotated, or "tipped", into the X-Y plane to produce a net transverse magnetic moment M t , which is rotating, or spinning, in the X-Y plane at the Larmor frequency.
  • M z nuclear magnetic resonance
  • NMR magnetic resonance
  • the excitation magnetic field near the Larmor frequency is produced by a separate coil placed adjacent the area of interest in the object being imaged. If an image of substantially the entire object is desired a large coil, often referred to as a "body coil” and extending around the entire object, is used to excite the spinning nuclei and receive the resultant electrical signals. In other situations when only a small portion of the object (such as the head of a medical patient) is to be imaged, a smaller radio frequency coil is placed about that portion of the object. The use of localized coils produce a more homogeneous excitation field within the desired portion.
  • radio frequency coil is cylindrical with a conductive loop at each end and axial conductive elements connecting the loops at periodic intervals around their circumference, as described in US-A-4,680,548 and US-A-4,692,705.
  • capacitors are placed in the end loops between adjacent axial conductive elements.
  • the coil formed an endless loop transmission matrix that is excited by two signals in quadrature to produce a rotating electromagnetic excitation field within the coil.
  • the RF excitation coils were symmetrical so that the electrical signals traveled uniformly around the coil to establish a highly homogeneous excitation field. Because of its shape and appearance, this coil structure is sometimes referred as a "birdcage".
  • Birdcage coils have been used in full body coils and in localized coils for the head of a patient. For this latter application the coil was slipped over the head until its end rested against the patient's shoulders. As the optimum imaging region is in the central portion of the coil due to field inhomogeneity near the ends, the ability of this type of head coil to image the neck of the patient is limited.
  • a radio frequency NMR coil comprising a first end loop comprising a first plurality of reactive devices coupled in series with a separate node between adjacent ones of the first plurality of reactive devices; a second end loop comprising a second pluarlaity of reactive devices coupled in series with a separate node between adjacent ones of the second plurality of reactive devices, wherein each of the second plurality of reactive devices has a reactance which is dependent upon where that respective reactive device is located in the second loop, and a plurality of conductive elements electrically connected between nodes of the first and second end loops, at least some of the plurality of conductive elements having different lengths than other ones of the plurality of conductive elements.
  • a radio frequency NMR coil comprising a plurality of periodic elements connected in series to form an endless loop, each of the periodic elements including a conductive element having two ends and a given inductance, wherein the inductance for at least some of the periodic elements differs substantially from the inductance of other periodic elements; a first pair of capacitance elements each having a first terminal connected to one end of the conductive element and a second terminal connected to a capacitance element of an adjacent periodic element; and a second pair of capacitance elements each having a first terminal connected to the other end of the conductive element and a second terminal connected to a capacitance element in an adjacent periodic element; wherein the first and second pairs of capacitance elements have capacitances determined by the inductance of the conductive element.
  • a radio frequency NMR coil includes a first end loop formed by a plurality cf conductive first segments connected in series with a node between adjacent segments.
  • a second end loop is disposed spaced from the first end loop along a central axis, and has a plurality of conductive second segments connected in series by a plurality of nodes.
  • a number of conductive elements of varying lengths are electrically connected between nodes of the first and second end loops. As the conductive elements vary in length, at least one of the end loops is non-planar.
  • One of a plurality of reactive devices is coupled between two sections of many of the first and second segments. Each such device has a reactance which varies depending upon where that device is located in first or second end loop.
  • some of the segments of a non-planar end loop are formed by coaxial cables having a central conductor connected between two nodes of the first end loop. The segments with coaxial cables do not have capacitors; whereas all the remaining segments of that end loop have capacitors.
  • the present invention seeks to provide a radio frecuency coil for an NMR imaging system, which coil can be shaped to conform to the anatomy of an object being imaged. Even though such conformance dictates that the coil has an asymmetric shape, the coil is designed to produce a relatively homogeneous magnetic field within an imaging volume.
  • FIG. 1 there is shown in block diagram form the major components of an NMR system in which the present invention can be used and which is sold by the General Electric Company under the trademark "SIGNA".
  • the overall operation of the system is under the control of a host computer system generally designated 100 which includes a main computer 101 (such as a Data General MV7800).
  • a main computer 101 such as a Data General MV7800
  • an interface 102 through which a plurality of computer peripheral devices and other NMR system components are coupled.
  • a magnetic tape drive 104 which under the direction of the main computer for archives patient data and images on tape. Processed patient data may also be stored in an image disc storage device designated 110.
  • image processor 108 The function of image processor 108 is to provide interactive image display manipulation such as magnification, image comparison, gray-scale adjustment and real-time data display.
  • the computer system is provided with a means to store raw data (i.e. before image construction) utilizing a disc data storage system designated 112.
  • An operator console 116 is also coupled to the computer by means of interface 102 and provides the operator with the means to input data pertinent to a patient study as well as additional data necessary for proper NMR system operation, such as calibrating, initiating and terminating scans.
  • the operator console 116 also is used to display images stored on discs or magnetic tape.
  • the computer system 100 exercises control over the NMR system by means of system control 118 and gradient amplifier system 128.
  • the computer 100 communicates with system control 118 by means of a link 103 in a manner well known to those skilled in the art.
  • the system control 118 includes several subsystems such as a pulse control module (PCM) 120, an array processor 106, a radio frequency transceiver 122, a status and control module (SCM) 124, and the power supplies generally designated 126 necessary to energize the components.
  • the PCM 120 utilizes control signals provided by main computer 101 to generate digital timing and control signals such as the digital waveforms which control gradient coil excitation, as well as RF envelope waveforms utilized in the transceiver 122 for modulating the RF excitation pulses.
  • the gradient waveforms are applied to the gradient amplifier system 128 generally comprised of G x , G y and G z amplifiers 130, 132 and 134, respectively.
  • Each amplifier 130, 132 and 134 excites a corresponding gradient coil in an assembly generally designated 136.
  • the gradient coils When energized, the gradient coils generate magnetic field gradients G x , G y and G z of the magnetic field in the same direction as the main polarizing magnetic field, wherein the gradients are directed in mutually orthogonal X, Y and Z axis directions of a Cartesian coordinate system.
  • the gradient magnetic fields are utilized in combination with radio frequency pulses generated by transceiver 122, RF amp 123 and RF coil 138 to encode spatial information into the NMR signals emanating from the region of the patient being studied.
  • Waveforms and control signals provided by the pulse control module 120 are utilized by the transceiver subsystem 122 for RF carrier modulation and mode control.
  • the transmitter In the transmit mode, the transmitter provides a radio frequency waveform modulated in accordance with the control signals to an RF power amplifier 123 which then energizes RF coil 138 which is situated within main magnet assembly 146 to produce the excitation field B 1 .
  • the NMR signals radiated by the excited nuclei in the patient are sensed by the RF coil and amplified by a preamplifier 139.
  • the NMR signals are amplified, demodulated, filtered, and digitized in the receiver section of the transceiver 122.
  • the processed NMR signals are transmitted to the array processor 106 for processing by means of a dedicated, unidirectional link 105.
  • the PCM 120 and SCM 124 are independent subsystems both of which communicate with main computer 101, peripheral system, such as patient positioning system 152, as well as to one another by means of serial communications link 103.
  • the SCM 124 includes means for acquiring information regarding patient cradle position, and the position of the moveable patient alignment light fan beam (not shown). This information is used by main computer 101 to modify image display and reconstruction parameters.
  • the SCM 124 also initiates functions such as actuation of the patient transport and alignment systems.
  • the gradient coil assembly 136 and the RF transmit and receiver coils 138 are mounted within the bore of the magnet utilized to produce the polarizing magnetic field.
  • the magnet forms a part of the main magnet assembly which includes the patient alignment system 148.
  • a shim power supply 140 is utilized to energize a shim coil associated with the main magnet and which are used to correct inhomogeneities in the polarizing magnet field.
  • the main power supply 142 is utilized to bring the polarizing field produced by the magnet to the proper operating strength and is then disconnected.
  • the patient alignment system 148 operates in combination with a patient cradle and transport system 150 and patient positioning system 152. To minimize interference from external sources, these NMR system components are enclosed in an RF shielded room generally designated 144.
  • the present invention relates to a novel RF coil 138 and Figure 2 illustrates one embodiment of this coil design for use in imaging the head of a patient.
  • the RF coil 138 has two electrically conductive end loops 161 and 162 spaced apart along a common central axis. These end loops may be circular, elliptical or another shape depending upon the desired geometry of the central opening of the coil in order to accommodate the part of the patient's body being imaged.
  • Sixteen conductive axial elements 164-179 interconnect the two end loops 161 and 162 at nodes to form a coil structure that resembles a cage, hence the name "birdcage" by which coils of this generic design are commonly referred.
  • the structure of the coil conductors defines a cylindrical imaging space within the coil.
  • RF coils according to the present invention can be constructed with a greater or a lesser number of axial elements.
  • the RF coil preferably is formed by applying conductors to a major surface of a tube, which has not been shown for ease of illustration.
  • axial elements 167-170 and 175-178 are shorter than the remaining elements creating a pair of notches in one end of the tubular coil.
  • the notches are sized to accommodate the shoulders of a patient being imaged in order for that end of the coil 138 to extend over the upper portion of the patient's thorax.
  • the extension of the coil over this anatomical portion improves imaging of the patient's neck and upper chest.
  • the first end loop is planar
  • the second end loop is non-planar forming a coil 138 that is asymmetrical.
  • the deviation of the coil from symmetry in all directions adversely affects the homogeneity of the RF field produced inside the coil.
  • the present coil compensates for the inhomogeneity by placing elements of different capacitance in the end loops 161 and 162.
  • Segments of the first end loop 161 between adjacent axial elements 164-179 are broken approximately at their midpoint.
  • a separate capacitor 181-196 bridges the break in each first loop segment.
  • Each capacitor 181-196 can comprise a surface mount capacitor or can be formed by overlapping the sections of the loop segment with dielectric material therebetween.
  • Segments of the second end loop 162 between adjacent axial elements 164-179 contain similar capacitors 201-216, with the exception of segments between axial elements 167-168, 169-170, 175-176 and 177-178.
  • each capacitor depends on the inductance of the adjacent axial elements.
  • the RF coil 138 is analyzed as a sixteen element periodic structure depicted in Figure 3 where one periodic section of the structure is designated 218. Note that each capacitor in Figure 2 is represented by two capacitors, one in each adjacent periodic element of Figure 3.
  • the inductance L and capacitance C were substantially equal at all locations around the coil and were selected so that the RF coil resonated at the Larmor frequency in two orthogonal resonant modes.
  • the same circuit analysis employed for those previous symmetrical MRI radio frequency coils is used to define initial values for the capacitors 181-196 and 201-216 in the present RF coil 138, as if that coil is symmetrical, i.e. does not have notch in end loop 162.
  • the capacitors, such as components 196 and 216, that are between two full length axial elements have this initial capacitance value.
  • the value of a capacitor in a periodic element with a shorter axial element corresponds to the variance of inductance of the axial element from the calculated initial value.
  • the value for each capacitor 181-196 and 201-216 in the coil 138 shown in Figure 2 is a combination of two capacitances in adjacent periodic elements in Figure 3.
  • Some of the capacitors can be adjustable to tune the coil 138 to resonate at the Lamor frequency in two orthogonal modes.
  • the RF coil 138 is excited by two radio frequency signals in quadrature from the RF amplifier 123.
  • One signal is applied via leads 226 across the capacitor 181 in the first end loop 161.
  • the other RF signal is applied via leads 228 and across the capacitor 185 spaced ninety degrees around the first end loop 161 from capacitor 181.
  • the quadrature excitation signals can be applied at other locations on the first loop 161 or to the second loop 162. In other embodiments of the present invention these signals could be applied to axial elements.
  • the segments of the second loop 162 between axial elements 167-168, 169-170, 175-176 and 177-178 are formed by coaxial cables 221, 222, 223 and 224 respectively, and do not contain capacitors.
  • Each coaxial cable 221-224 has a center conductor connected across ends of the two adjacent axial elements, for example the center conductor of cable 221 is connected in the second end loop 162 between axial elements 167 and 168.
  • the outer coaxial shield of each cable 221-224 is unconnected.
  • capacitors 183, 185, 191 and 193 are one-half the value calculated as described above, since the parallel capacitor in the second end loop 162 has been removed.
  • the coaxial cables 221-224 restore some of the field homogeneity.
  • Current flowing through the center conductor of these coaxial cables produces a current in the opposite direction in their shields even though the shields electrically float.
  • the reverse current in the shields of the coaxial cables contributes to the imaging field and the current in the center conductor has minimal contribution.

Claims (9)

  1. Bobine de radio fréquence pour imagerie par résonance magnétique comprenant:
    une première boucle d'extrémité (161) comprenant une première pluralité de dispositif réactifs (181-196) couplé en série avec un noeud séparé entre ceux adjacents des dispositifs de ladite première pluralité de dispositifs réactifs (181-196);
    une deuxième boucle d'extrémité (162) comprenant une deuxième pluralité de dispositifs réactifs (201-216) couplés en série avec un noeud séparé entre ceux adjacents des dispositifs réactifs de la deuxième pluralité de dispositifs réactifs (201-216), dans lequel la deuxième pluralité de dispositifs réactifs (201-216) présente une réactance qui dépend de l'endroit où le dispositif réactif respectif se trouve dans la deuxième boucle (162), et
    une pluralité d'éléments conducteurs (164-179) connecté électriquement entre des noeuds des première et deuxième boucles d'extrémité (161, 162), au moins certains éléments conducteurs de la pluralité d'éléments conducteurs (167-170, 175-178) ayant des longueurs différentes de celle d'autres éléments conducteurs de la pluralité d'éléments conducteurs (171-174, 179, 164-166).
  2. Bobine de radio fréquence pour imagerie par résonance magnétique selon la revendication 1, dans laquelle chaque dispositif réactif des première et deuxième pluralités de dispositifs réactifs (181-196, 201-216) comprend un condensateur.
  3. Bobine de radio fréquence pour imagerie par résonance magnétique selon la revendication 1 ou 2, dans lequel chaque dispositif réactif (181-196, 201-216) des première et deuxième pluralités présente une capacitance déterminée par l'inductance de ceux des éléments conducteurs (164- 179) qui sont connectés aux noeuds auxquels se connectent le dispositif réactif respectif.
  4. Bobine de radio fréquence pour imagerie par résonance magnétique selon l'une quelconque des revendications précédentes, dans laquelle la deuxième boucle d'extrémité (162) comprend, en outre, un câble coaxial (221-224) comportant un conducteur central connecté en série avec la deuxième pluralité de dispositifs réactifs (201-216) et entre deux noeuds de la deuxième boucle d'extrémité (162).
  5. Bobine de radio fréquence pour imagerie par résonance magnétique selon la revendication 4, dans laquelle le blindage extérieur du câble coaxial (221-224) n'est pas connecté.
  6. Bobine de radio fréquence pour imagerie par résonance magnétique selon les revendications 4 ou 5, dans laquelle des segments de la deuxième boucle d'extrémité (162) comportant le câble coaxial (221-224) ne contient pas de dispositif réactif.
  7. Bobine de radio fréquence pour imagerie par résonance magnétique selon l'une quelconque des revendications précédentes, dans lequel la première boucle d'extrémité (161) est plane et la deuxième boucle d'extrémité (162) n'est pas plane.
  8. Bobine de radio fréquence pour imagerie par résonance magnétique selon l'une quelconque des revendications précédentes, dans laquelle la pluralité d'éléments conducteurs (164-179) sont espacés périodiquement autour des première et deuxième boucles (161, 162).
  9. Bobine de radio fréquence pour imagerie par résonance magnétique comprenant une pluralité d'éléments périodiques (218) connectés en série de manière à former une boucle sans fin, chacun des éléments périodiques comprenant:
    un élément conducteur comportant deux extrémités et présentant une inductance donnée, dans lequel l'inductance pour au moins certains des éléments périodiques diffère substantiellement de l'inductance des autres éléments périodiques;
    une première paire d'éléments doués de capacitance comportant chacun une première borne connectée à une des extrémités de l'élément conducteur et une deuxième borne connectée à un élément doué de capacitance d'un élément périodique adjacent; et
    une deuxième paire d'éléments doués de capacitance comportant chacun une première borne connectée à l'autre extrémité de l'élément conducteur et une deuxième borne connectée à un élément doué de capacitance dans un élément périodique adjacent;
    dans laquelle les première et deuxième paires d'éléments doués de capacitance présente une capacitances déterminée par l'inductance de l'élément conducteur.
EP93303931A 1992-05-22 1993-05-20 Bobine asymétrique de radio fréquence pour l'imagerie par résonance magnétique Expired - Lifetime EP0571211B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US07/887,634 US5309104A (en) 1992-05-22 1992-05-22 Asymmetric radio frequency coil for magnetic resonance imaging
US887634 1992-05-22

Publications (2)

Publication Number Publication Date
EP0571211A1 EP0571211A1 (fr) 1993-11-24
EP0571211B1 true EP0571211B1 (fr) 1998-03-11

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US (1) US5309104A (fr)
EP (1) EP0571211B1 (fr)
JP (1) JP3340791B2 (fr)
DE (1) DE69317327T2 (fr)
IL (1) IL105664A (fr)

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US5998999A (en) * 1996-12-12 1999-12-07 Picker International, Inc. Volume RF coils with integrated high resolution focus coils for magnetic resonance imaging
US6169401B1 (en) 1998-11-25 2001-01-02 Picker International, Inc. Flexible open quadrature highpass ladder structure RF surface coil in magnetic resonance imaging
US6313633B1 (en) * 1999-12-27 2001-11-06 General Electric Company Magnetic resonance imaging head coil
DE10109489B4 (de) * 2000-03-31 2009-03-05 RAPID Biomedizinische Geräte RAPID Biomedical GmbH Spinresonanzmeßgerät mit einem statischen Magnetfeld
US6946836B2 (en) * 2000-04-25 2005-09-20 Kabushiki Kaisha Toshiba Magnetic resonance imaging involving movement of patient's couch
AUPR005900A0 (en) * 2000-09-11 2000-10-05 Nmr Holdings No. 2 Pty Limited Asymmetric radio frequency coils for magnetic resonance
AU783991B2 (en) * 2000-09-11 2006-01-12 Nmr Holdings No. 2 Pty Limited Asymmetric radio frequency coils for magnetic resonance
WO2007103953A2 (fr) 2006-03-09 2007-09-13 Insight Neuroimaging Systems, Llc conception de bobines à microruban pour appareils d'IRM
JP4879811B2 (ja) * 2007-04-23 2012-02-22 株式会社日立製作所 コイル装置及びそれを用いた磁気共鳴検査装置
KR102277899B1 (ko) 2014-08-11 2021-07-15 삼성전자주식회사 Rf 코일부 및 이를 포함하는 자기공명영상 시스템
SG11202109090SA (en) 2019-02-22 2021-09-29 Promaxo Inc Pseudo-birdcage coil with variable tuning and applications thereof

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DE4035844A1 (de) * 1990-11-10 1992-05-14 Philips Patentverwaltung Quadraturspulenanordnung
DE69221835T2 (de) * 1991-12-11 1998-03-05 Philips Electronics Nv Magnetresonanzgerät mit einer Vogelkäfig-RF-Spule

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DE69317327D1 (de) 1998-04-16
JP3340791B2 (ja) 2002-11-05
IL105664A0 (en) 1993-09-22
EP0571211A1 (fr) 1993-11-24
IL105664A (en) 1995-03-15
JPH0654822A (ja) 1994-03-01
DE69317327T2 (de) 1998-10-08
US5309104A (en) 1994-05-03

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